Imagine that instead of a planet orbiting around a star – like the Earth around the Sun – another star was orbiting it. And instead of one star circling the other, the two were orbiting each other, tumbling along like a couple of acrobats. Such stars exist and are called binary stars.
People have known about binary stars for a long time. Clear back in 1650, Giovanni Riccioli noticed that Mizar, one of the stars in the Big Dipper, had a close friend and was, actually, a double star. Many more examples have been found.
What if binary stars come near a black hole? Probably they would both get sucked in. But what if one was captured by the powerful gravitational pull of a black hole, but the other, like a baseball released by a pitcher or a stone from a slingshot, was flung away?
This idea was proposed in 1988 by scientist Jack G. Hills. Most stars in the Milky Way have a speed of around 224,000 miles per hour. (Our Sun, bless its speedy little heart, is going about 448,000 mph) Hills showed mathematically that if a binary star were flung by the supermassive black hole at the center of the Milky Way, it would go much much faster, even exceeding the escape velocity of the galaxy. In other words, the star could be flung right out of the Milky Way.
In response to Hills’ work, astronomers began searching for super fast-moving stars. Seventeen years later, in 2005, they found one. The star is called SDSS J090745.0+24507, but was quickly given the nickname, the Outcast Star. It’s speed was estimated to be around 1.6 million miles per hour.
The Milky Way is so huge that a star, even one traveling at 1.6 million miles an hour, would take a long time to get out of town. Let’s put the situation in perspective.
A light year is the distance light travels in one year, around 6 trillion miles. The Milky Way is estimated to be 100,000 light years in diameter, which is pretty darn big.
Since 2005 when the Outcast Star was discovered, how far has it traveled? In 19 years there are 166,554 hours. Multiply that by 1.6 million miles per hour and you get 266 billion miles. That’s less than five percent of one light year. So the Outcast Star won’t be an actual outcast for quite awhile. Which means that we here on Earth will have plenty of time to study it.
So far, astronomers have discovered about 20 hyper-velocity stars. The Outcast Star, the first one they found, has a speed of 1.6 million miles per hour. A star called US 708 is traveling at 2.7 million miles per hour. A star called S5-HVS1 was clocked at 3.9 million mph. But hold onto your hats. Recently, a star, J0927, was found to have an estimated speed of 5.1 million mph. I hereby nickname it Zip-a-Dee-Doo-Dah!
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